In clinical trials with alcohol-dependent patients, the opioid antagonist naltrexone reduced alcohol craving, increased abstinence rates, and decreased relapse rates. However, the population contains a large number of heavy drinkers that are not yet alcoholic. Placebo-controlled studies of naltrexone have not been conducted with heavy drinkers. In addition, naltrexone increases adrenocorticotropin (ACTH) and cortisol which may mediate aversive subjective effects and predict therapeutic outcome of naltrexone treatment. The purpose of this study is to determine naltrexone's efficacy in enhancing completion rates and drinking outcomes for heavy drinkers enrolled in the DrinkWise Counseling Program at the University of Michigan. We also want to determine if the ACTH and cortisol response to naltrexone predicts therapeutic outcomes, and whether that neuroendocrine response is correlated with subjective mood states. The subjects will be male and female DrinkWise clients informed of the study during their first DrinkWise visit. Prior to the second DrinkWise visit, potential subjects will meet with a researcher during which screening tests will be performed to determine subject eligibility for the study. After screening, subjects will be randomized to receive daily doses of naltrexone (50 mg tablet) or placebo for 6 weeks. The first dose of study medication will be administered at the General Clinical Research Center (GCRC) on the day of the second DrinkWise visit. At the GCRC, subjects will receive an intravenous catheter for 3.5 hours to draw blood for neuroendocrine analysis. Subjective questionnaires will be administered during the blood draw session. Following the blood draw session, each subject will be sent home with a supply of study medication. Subjects will attend normal DrinkWise meetings and be assessed by research staff every 2 weeks for a total of 6 weeks and then re-assessed at 3 and 9 months after termination of study medication. We predict that greater program completion rates and better drinking outcomes will be observed in the naltrexone-treated subjects compared to the placebo-treated subjects. We also hypothesize that large increases of ACTH and cortisol in response to naltrexone will be correlated with negative subjective mood states and better treatment outcomes. The results of this study will be useful for determining the patient population that may benefit the most from therapeutic treatment with naltrexone.